Japanese Patent Application Publication No. 2006-2304774 discloses a method for detecting red blood cells infected with the malaria parasite (referred to as ‘malaria infected red blood cells’ below). In this detection method, a measurement sample is prepared by mixing a blood sample with reagent that can partially lyse the cell membrane of red blood cells so that a staining dye holding malaria parasite inside and that contains a stain that stains DNA more strongly than RNA, then the measurement sample is introduced to a flow cytometric flow cell, the measurement sample flowing through the flow cell is irradiated by light, the scattered light and fluorescent light given off by the measurement sample are detected, and a scattergram is created with the scattered light intensity and fluorescent light intensity as the two axes.
In red blood cells infected by the ring-form type of malarial parasite, there are red blood cells containing a single malarial parasite (referred to as ‘single-ring form’ below) and red blood cells containing two or more malarial parasites (referred to as multi-ring form’ below), with different amounts of DNA contained in the single-ring form and multi-ring form. Also, in falciparum malaria, the proportion of multi ring-form tends to be high. Focusing on this, the detection method disclosed in patent reference 1, separately detects single-ring form and multi-ring form that appear in regions of different fluorescent light intensities in a scattergram, and determines whether it is falciparum malaria based on the number of dots (number of cells) present in the multi-ring form region.
Sometimes the multi-ring form may not appear in falciparum malaria. In the detection method disclosed in Patent reference 1, there is a possibility that an accurate determination may not be made for a measurement sample using the number of multi-ring forms. It is therefore desirable to improve the determination accuracy regarding whether a measurement sample is infected by Plasmodium falciparum. 